For reasons of comfort and safety, electrically heatable vehicle seats are used in modern vehicles. The driver's seat, as well as the other seats in the vehicle, can be so arranged that they can be heated by means of special heating elements in the form of electrically conducting wires which can be placed in each seat in the form of a heating coil. Such a heating element is normally placed in the back-rest and the cushion of each seat when the seat is manufactured. The heating element is furthermore connected to a current feeding unit which delivers current. In this manner, the heating element can be heated to a suitable temperature.
A problem in previously known heating elements is due to the desire for each seat to have a carefully adjusted temperature on its surface, i.e. on that surface which is in contact with the person travelling in the vehicle. For this purpose, the temperature of the heating element can be controlled by means of a temperature sensor which is arranged close to the heating element, and which is connected to a central control unit. Using the temperature sensor and the control unit, the present temperature can be detected. The control unit also comprises current feeding circuits which, for example, can be based on transistor or relay technology for feeding current to the heating element. In this way, the central control unit is arranged to feed a certain current to the heating element until a certain desired set value for the temperature is reached. The adjustment of this set value can be done either by means of fixed resistances or by means of an adjustable potentiometer which is adjusted by the person travelling in the vehicle.
Using the above described control method, current can be delivered to the heating element until the central control unit indicates that the set value has been reached. When this happens, the control unit will cut off the power feeding. This will cause the heating element to successively cool down. When the heating element has cooled so that its temperature again is below the set value, the power feeding will be resumed. In this way, the temperature control will continue as long as the system is operative.
Although this previously known system normally provides a reliable heating and temperature control for a vehicle seat, it however has certain drawbacks. One such drawback is due to the fact that the heating element is normally fixed in the vehicle seat when manufactured, and the heating element is adjusted according to a certain "normal seat", i.e. a seat which is defined in advance with a certain given design, upholstery, etc. The seat in this way comprises a heating element for heating the surface of the seat to a certain set temperature. However, when assembling the vehicle the manufacturer might choose to provide the seat in question with, for example, a completely different upholstery, for example a considerably much thicker upholstery than that of said "normal" seat, i.e. an upholstery which differs from that which the temperature control originally was intended for. The temperature value which is detected by the temperature sensor will reach the set value when the heating element has reached the desired temperature, but since an abnormally thick upholstery then is used on the seat, the temperature which is present on the surface of the seat will be too low. This situation thus creates an undesired deviation in the temperature control.
Another drawback of previously known systems for heating seats is that they normally comprise relatively complex and extensive electronic circuits for temperature control and current feeding to the heating element. There are, for example, PCB-solutions where a multitude of components are arranged on a certain circuit board. This might create problems as regards the reliability of the heating system.
A further drawback of previously known systems is that malfunctions can occur, for example in the heating element, without information being transferred to any central unit. For example, short circuits and breakdowns in the wire can occur in the heating element without being detected in a secure and efficient manner.